All-Atom Simulations Uncover Structural and Dynamical Properties of STING Proteins in the Membrane System

Recent studies have shown that the stimulator of interferon gene (STING) protein plays a central role in the immune system by facilitating the production of type I interferons in cells. The STING signaling pathway is also a prominent activator of cancer-killing T cells that initiate a powerful adaptive immune response. Since biomolecular signaling pathways are complicated and not easily identified through traditional experiments, molecular dynamics (MD) has often been used to study structural and dynamical responses of biological pathways. Here, we carried out MD simulations for full-length chicken and human STING (chSTING and hSTING) proteins. Specifically, we investigated ligand-bound closed (holo) and ligand-unbound open (apo) forms of STING in the membrane system by comparing their conformational and dynamical differences. Our research provides clues for understanding the mechanism of the STING signaling pathway by uncovering detailed insights for the examined systems: the residues from each chain in the binding pocket are strongly correlated to one another in the open STING structure compared with those in the closed STING structure. Ligand-bound closed STING displays ∼174° rotation of the ligand-binding domain (LBD) relative to the open STING structure. The dynamical analysis of residue Cys148 located in the linker region of hSTING does not support the earlier hypothesis that Cys148 can form disulfide bonds between adjacent STING dimers. We also demonstrate that using the full-length proteins is critical, since the MD simulations of the LBD portion alone cannot properly describe the global conformational properties of STING.

近期研究表明，干扰素基因刺激因子（stimulator of interferon gene, STING）蛋白通过介导细胞内I型干扰素的产生，在免疫系统中发挥核心作用。STING信号通路同时也是启动强效适应性免疫应答的杀癌T细胞的重要激活因子。由于生物分子信号通路结构复杂且难以通过传统实验手段识别，分子动力学（molecular dynamics, MD）技术常被用于研究生物通路的结构与动力学响应。本研究针对全长鸡源与人类STING（chSTING和hSTING）蛋白开展了MD模拟。具体而言，我们在膜系统中研究了STING的配体结合闭合态（holo，全合状态）与无配体开放态（apo，脱辅基状态）两种构象，并对比二者的构象与动力学差异。本研究通过对上述受试系统的深入解析，为理解STING信号通路的作用机制提供了线索：相较于闭合态STING结构，开放态STING结构中结合口袋内每条肽链的残基之间具有更强的相关性。配体结合闭合态STING的配体结合结构域（ligand-binding domain, LBD）相较于开放态STING结构，呈现约174°的旋转。针对人类STING连接区域内的Cys148残基开展的动力学分析，并不支持此前关于Cys148可在相邻STING二聚体之间形成二硫键的假说。本研究同时证实，使用全长蛋白至关重要，因为仅针对LBD结构域开展的MD模拟无法准确描述STING的全局构象特性。